I too was very p'd off when I got breast cancer as I spent a lot of time and effort last year getting really fit! Great to hear Roadrunners account as I'm planning to try and run through treatment too.
I ran between diagnosis and my mastectomy (Christmas) and a second op at the end of Jan (axillary clearance). I started running again 3 weeks ago (5k three times per week) and I now feel normal again. I start chemo next week (FEC-T, then Herceptin & radiotherapy) and I plan to try and run/walk throughout.
When I was at the stage you are at now I did feel a bit low on occasion and I was particularly anxious waiting for biopsy results. I've got young children and an aggressive cancer - you can't help but have a few doom and gloom moments. As I've moved through treatment I've gradually felt more and more in control. Getting back into my normal exercise routine has really helped.
There have been trails on the benefit of exercise and one I read up on said that out of the two test groups the group that did high cardio exercise 3 times a week were 60% less likely to have cancer recurr than the less active group!
I ran all through treatment, and I recommend it both for physical and emotional wellbeing. I had a mastectomy, chemo, rads, and herceptin. Immediately after surgery I started walking around the block, building up the distance. I think I started running again three weeks after surgery. After that I never stopped. I got more and more tired through chemo (the FEC was OK, but Tax takes it out of you, it makes the major muscles very tired). I ran the race for life after the 6th of 8 chemos. Then once into herceptin it gradually got better, then got worse again as the cumulative effect of the herceptin took its toll.
I was fit and healthy before, so understand where you are coming from. But all research says exercise reduces the incidence of BC and reduces the recurrence rate so I won't stop running!
FYI - I am now 8 years on from diagnosis.