Well,
I wrote a long message yesterday in response to your questions about doing a 1/2 marathon. The connection I had with te internet went down, I lost what I wrote, so- I am just going to summarize: I would definetly sign up- Having a goal to work for is a good thing. If you aren't up to running, as you said- walk part of it. DOing something is better than doing nothing.

I just started running about 8 months ago- never was a runner and have done a few 5 k's, 3 -10k's and am scheduled for my first 1/2 in 2 weeks.
I never had any idea what to expect-Not being part of any formalized running competitions before, I never could have imagined the "competitive" aspect I would have in this area and the satisfaction I felt when I completed each race.(now I am hooked)It as also been the BEST form of toning and weight loss assistance I have had.
For me- my surgery has been filled with complications and problems: running for me is a chance to "escape those negative aspects" plus being able to see a "finish Line". I haven't yet found the end with my health issues, but, it is nice to work towards improving my time, completing the race, and seeing an actual "finish" line and being able to cross it.
I say--- Go for it-- Good luck with yoru surgery, your success, and your future life.
God Bless,
Amy H