I vote for a mixture as well. There has been lots of research done that shows intervals being very very beneficial for most folk. Of course this is for general populous.
Since I can only walk now, I walk at 3mph, then 4mph, then 3.5 mph, then rinse and repeat. This makes it quite fun. If you are outside you can leisurely walk for 2-3 minutes and speed walk for 1 minute, if comfortable, up the speed walking times. I find that intervals take the boredom out of walking and I can go for a while. Heck, when outside, you can do whatever you want, though, different intensities make it fun. I wear my wrist heart monitor which lets me measure my heart-rate and monitor things for BPM limits, avg BPM, as well as speed with which my BPM lowers back to exercise zone. When I see my hear-rate taking longer to get back to my zone, I just about call it a day. Though, so far I just call it a day at 1 hour or there abouts as that is my current time-limit for exercise until week 3.