Active not reactive?

He can be proactive if he has a goal to work towards that he actually pursues.

Being reactive doesn't mean being inactive. It just means that something exterior to the character is directing his activities; the character's "goal" comes into existence because some other force/character made that goal necessary, and it's more of an ad hoc goal (like getting away or blocking a blow.) Even if a character actually fights a foe, he can still be acting reactively. For example, if he's going about his business and he's suddenly attacked by a mysterious being, whether he runs or, if he can't run, fights, he's being reactive.

Being proactive would be: hunting down that mysterious being who is going about his/her business elsewhere.

I think those prefixes point at the significant difference. When is the goal decided? Before the action starts...or after, as a consequence of others' actions that have already occurred?

So in your example, if all Mark is doing is running away from fighting in the army, then he's being reactive. I.e., whatever actions he is taking are all about getting away from that summons to duty that has already occurred.

To make him proactive, you'd need to give him a goal of his own. Getting away is only a goal because that summons to battle came first; it is reactive. But if you give him, instead, something else to pursue, and he can remove himself from having to fight in the army while pursuing this other thing, then he'd be proactive and avoiding battle would be a mere bonus.

 

I think the long story short version is: running away is fine but at the same time, it's best if the character is running to something.

Cowards are a hard MC, no doubt about that. Even as POV character, it's a rough go.

 

Just a note. Early in a novel or story, characters might often be acting reactively. An inciting event happens; they react. Usually by about somewhere in the second act, MCs should probably begin to transition to being far more proactive if they start out the book being quite reactive. (Of course, characters that are proactive from the very beginning can be, and probably are, more interesting on the whole.)

Also, if the situation of the story requires them to be reactive early, this doesn't mean that they can't also be proactive in other ways. Even if they are having to react to a villain's actions or other events, they can be proactive in pursuing a love interest, a side quest, and so forth, and this can show the character has the capacity to be a proactive character or have a proactive personality.

Here's a great Writing Excuses podcast covering the topic: Writing Excuses 9.32: Adjusting Character Proactivity | Writing Excuses.