Not sure what to do and I need your help desperately

I'm really good at beginning things. I've nicknamed myself, "A million
little hobbies". I start lots of things. I have a hard time finishing
things I've started.

Knitting. Check.
Crochet. Check.
Quilting. Check.
Triathlon. Check.
Grad School. Check.
Swimming. Check.
Biking. Check.
Running. Check.
Writing. Mothering. Wife-ing. Vegan-ing.
Check. Check. Check. And Check.

The list could continue. I'm now faced with a serious dilemma and I
really don't know what to do. I need suggestions.

I must complete something. At least one thing and I need to chose which one.


Thesis
I have to reconstruct a good portion of my thesis. That takes time. A
lot of time. Especially on weekends. I could use all the time I spend
running to write and re-write and re-construct.

Running
And yet I committed (to myself) to doing California International
Marathon (CIM). Really committed. I've been training well and
consistently up until last month when I was sidelined with
injury/illness. I've lost a lot of training.

I'm trying to figure out if I should give up the CIM idea now. Of
course there is a part of me that wants to complete it since I started
it. But I'm also afraid if I continue, I'll
a) hurt myself trying to get back up to where I am supposed to be in my training
b) not be able to concentrate on my new thesis/adviser/etc.
c)burn out

What is your opinion? Can I do both? Should I try?