What is excellence? Part of the answer lies in an analysis of what it means to excellently engage in a practice. (You can engage in a practice without excellently engaging so; the point of the analysis wouldn't be to find out what it means to engage in a practice, but precisely what it means to engage in it excellently.) This is for another time, however; let's simply assume we know well enough what that means.
Even then, it is only part of the answer, because I believe excellence is also rightfully attributed to persons, not just to single episodes of a person's actions, thoughts, and feelings; and we need to say something about the relationship between a person's excellently engaging in practices and the ascription of excellence to her as a person.
What makes a person excellent is more than that this person frequently performs excellently in practices; excellence in a person is not simply her mostly thinking, feeling, and acting excellently; to put the point differently: to attribute excellence to a person is more than just a shortcut for 'when she engages in practices in her life, she often (or mostly, or typically) does it in an excellent way, she is frequently excellent in what she does'.
Rather, I think that it's the other way round: excellence in engaging in practices flows from someone's excellence as a person; of course, it then also reflects on that person, and makes us see and admire her as excellent. But excellence in engaging in practices is not constitutive of excellence in persons, it is how that excellence manifests itself (shows and articulates itself) in that person's actions, views, and feelings.
Engaging excellently in a given episode of a practice never exhausts the excellence of the person who engages in that episode, there is always more to that person than is revealed in a single episode. There is also always more to a person than is revealed even in a series of episodes of a given type. Excellence as a person unifies excellence in engaging in various episodes of diverse types. But even if you take the sum total of all episodes in which a person has (yet) engaged and extract how excellent the performance of that person has been in those episodes, you wouldn't have arrived at the excellence of that person. You would also have to consider all episodes in which that person might have been, and how she would have acted, thought, and felt then.
By now it would seem that we have arrived at a conception of excellence in a person which takes it to be equivalent to a structure of dispositions: the dispositions to act, think, and feel in any given episode of the various kinds. But even this wouldn't be enough: for these dispositions (and this total structure of dispositions) will inevitably change all the time, and it will change precisely (if only minutely) every time a person engages in a practice. There is no fixed structure of dispositions to act; there is a dynamic structure which continuously changes.
This has two important consequences. The first consequence is that excellence as a person can neither be determined by the actual episodes of that person engaging in practices nor by all the possible episodes of such engaging. It cannot be determined by the space of all possible situations in which a person might find herself and the way she acts, thinks, and feels in those situations: this is too wide; it leaves out the crucial ingredient of which episodes actually happened and shaped the structure of dispositions to engage in practices. And it cannot be determined by the concrete set of actual situations (so far) in which that person found herself and in fact engaged, that is, it cannot be determined by the factual history of that person: this is too narrow, for it leaves out ways she might have responded to circumstance which simply by historical accident didn't occur. Thus, the first consequence can be put this way: when we're looking to figure out what excellence as a person means, we have to consider both the entire space of possibilities and the actualities that have in fact obtained. In other words, the way events have unfolded in reality has a part in determining the excellence of a person (as a person).
The second consequence is that excellence of a person is something that develops. Every time you respond to how events unfold in an excellent manner will move you towards your excellence as a person. There is no fixed structure of dispositions; there is an ever-changing structure as long as you live (or, more precisely: as long as you live your life, by acting, thinking, and feeling in response to the world around you; this might leave out some merely vegetative states, although the borders here might be fuzzy). Shaping your ways of acting, thinking, and feeling so that they become more excellent means thus to become more excellent as a person. Conversely, letting yourself go, taking your own weakness as given and not improving on them will move you, on the whole, away from excellence.
Both trends are self-perpetuating. The reason why they are self-perpetuating has to do with the first consequence above: reality itself (by the more or less random chain of events in which you find yourself partaking) plays some role in shaping your excellence. Thus when you are moving yourself towards excellence as a person, you will after a while find yourself supported by how events run: you will gain, as I put it in my book, momentum from reality. If, on the other hand, you let yourself go and forgo excellence in what you do, you will sink further and further towards weakness, and the gravitational force of events will compound that effect. Both going for excellence and refraining from that quest have a self-perpetuating characteristic that comes from the role which reality plays in your engaging with it.