friendtoanimals, I want to welcome you to the forum. You certainly have been through a lot! It is common, however, for a person with one autoimmune disease to have others. I have to say that I've not heard of anyone having a trigger of exercise. However, we do have quite a few people here with AIH so maybe one of them can respond.
Considering what you have experienced thus far, I think it would be wise to listen to your doctor for the time being. No marathons or half-marathons! Then, after several weeks, try walking again--no running. I also want to suggest to you something else for anxiety: Yoga. You will be stretching your body gently and calming your body and mind without stressing your liver. I do know that stress is not good for liver disease, be it emotional or physical. You might also consider Tai Chi and bicycling (no racing!). The idea here is to find an alternative to running/brisk walking that will not cause problems for your liver.
I, too, have AIH. I haven't heard of it being triggered by exercise. Stress (therefore anxiety) can be a trigger and that's why we are supposed to avoid heavy periods of stress (as much as possible). As per exercise, we are encouraged to remain as active as we can since there is an increased risk for fatty liver disease. Perhaps the type of exercise you have been doing (marathons are no easy walk in the park ) are too stressful for your body. Everyone is different. Other forms of exercise should be fine as Connie suggested yoga and walking - not power walking since you appear to be more sensitive. I do spin and go to the gym when I can. I also do the breast cancer 3-day which is a 60mile walk over 3 days but we have all day to do it so it really isn't very taxing provided we've trained enough. I haven't had any restrictions put on me. I did gain weight and still have to lose some but I haven't put a huge amount of pressure on myself to do it. I only have 12 lbs to go to pre sickness weight. It should be gone by summer!
You said "ramp up of Imuran". So you are currently on it? How much? I have HSV but was diagnosed after the AIH and it hasn't affected anything...knock on wood! I am currently on 100mg of Imuran and no more pred. Got off that in Dec 2008. I was on it a total of 22 months. I was on 5mg of pred for 11 months and then before my final wean (which is the slowest of all the pred weans) I boosted up to 100mg Imuran. The 5mg to 0 wean took 2 months. It has to be the slowest to allow your adrenals time to kick back in to work. I haven't had any problems with the Imuran...except for possible cholestasis but I opted to treat that separately with Urso and not lower my Imuran dose as I didn't want to risk a flare. There is a test they can do to find out how much or if any Imuran your body can tolerate. It measures TPMT (wikipedia that and you'll find out more).
Are you seeing a hepatologist? You should be! They have a specialty in liver disorders which is better than seeing a GI unless you don't have access to a hepatologist. You should also have a baseline bone density test and be taking calcium with Vit D as pred causes bone loss.
Keep up posted,