When you are fully well, you can go back to your previous levels of exercise, but think of Lyme as the bacterial version of a broken leg
-- hobble gently as much as you can tolerate without overdoing it, and
when you are fully well, you'll be back to marathons.
Light to moderate exercise is good for all the usual reasons, and in Lyme, it's just a matter of not overdoing it. When you are fully well, you'll work back up to your previous levels of activity. Patience, grasshopper; patience.