I have to agree with Rosie - I've been trying to get back to walking like I used to, and everytime I "push" because I have some goal in mind, or it just feels good to be outside, or I just hate giving up - that push feels like a big shove later on, and I end up worse off than I was before - sidelined again and starting over. There are days when even trying to get to the end of the block (let alone around it) is too much (hate those days) but I've learned to really pay attention. If I'm unsure - I'll go ahead and start out but at the slightest hint of it being too much, I'm done.
I walk at night with my husband in our neighborhood - usually after dinner. It was our practice for years and years before I had the AI problems. We used to do 2-3 miles every night. Now I prefer that time because I don't have to worry about the sun exposure and I actually feel better in the evening than I do in the morning or afternoon. I had a physical therapist tell me I'd get the best benefit from exercise if I could schedule it for the time of day when my body feels most energized. I'm trying to get myself out there in the afternoon for a short block or so, but I prefer going with someone. I think I get a little anxious by myself due to all those times I made myself go and then went too far and then wasn't sure I could make it home - horrible feeling. Pushing just hasn't paid off for me.
Anyway - I'm right where you are, Redrose, trying to get up to 1/2 hour of walking in everyday. But I'm still at 15-20 min. on a good day. And I go at a "comfortable" pace - which means different things on different days. At least it's something and any movement counts in my book. So I don't worry about doing too little - just that I keep trying. You can build on whatever small steps you begin with. Good Luck to you - I really think walking is the best exercise overall - and progress is progress, no matter how small the increase, or how long it takes to accomplish it.