short answer: part time sucks. long answer: part time sucks. i did a variant of this (school for part of it, work for another) for 3 years with my first company, and it was frustrating because you miss opportunity after opportunity because you move so slowly, and everything just turns out mediocre.

so if you have to do your startup with a 40-50hr/wk job, do it as briefly as possible until you can prove to yourself the worth of the idea and then make the jump. and this may sound strange, but get yourself in decent physical shape so that you have more stamina and aren't exhausted after a normal 8-9 hour work day. in my case i had way more energy/focus if i was running/lifting regularly (and frittered around/needed more downtime if i wasn't.)

i'm going full time on my idea at the end of this week and have a lot more momentum because 1) other people will be working on it with me and 2) i will be able to focus 100% and do things like pull all nighters when necessary and basically not have any other constraints in the way of getting things done.