I am wanting to start the C25k tomorrow. I am trying to actively lose weight and I want to exercise a lot, since I have the time. Yet the C25k is only 20 minutes to begin.

Would it be bad if I did the C25k part, then did additional walking? I usually do more anyways....I mean, I've even done a half marathon before. I just need to now start working towards being able to jog enough.

Complete with proper warm ups, C25k is 30 minutes to start out, but by all means if you want to keep walking, do so!

I make the c25k last longer by doing extra intervals of walking and jogging...

Extra walking sounds fine. I always do a cool down walk at the end. ... or you could just skip ahead to week 2 or 3 if week 1 sounds too easy.

When I first started trying to lose weight, I was too out of shape to even make it through week 1 of the C25K, so I started walking instead. I walked several miles a day for many months before trying the C25K again. When I finally felt comfortable trying to run again, I hated to give up my walking, so I just added the C25K program to my daily workout. Most days I was walking 3 miles on my lunch at work, 2-3 miles with my dog after work, then 3 days a week also doing the C25K program in the evening. Or I would do the C25K program, get done, run back to my apartment to get my dog, then do her 2-3 mile walk as my cool down. I now do all my regular walking in addition to running 3 miles every other day and feel great when I get done with everything. If your body seems okay with it, then I would go for it. Just pay attention to how your body responds and scale back a bit if you start to have problems.

Yes, I'm in the middle of C25K and I often do an extra 1-2 miles of walking once I'm done with the actual C25K part. I just listen to my body and keep my walking pace/distance in line with what I have the energy to do.