I think Michele is the Enabler-in-Chief
As Michele said, I walk/run outside with my doggies and I LOVE Cardio Coach. I have tried ITread but I just hated it, in fact, about 20 minutes into it I just ripped the headphones out of my ear and was ready to trample on them
I think all CCs are great to run outside, you just need to modify. I do hills but I also substitute them for faster walks or runs, or even sprints. No walking lunges for me, I hate lunges and it would be really hard with 5 dogs in tow
I think the best one with limited hills is Vol. 3, at 45 min it is also a little shorter than 5 and 7. 1 and 2 are also shorter and you can substitute the hills with speed. That being said my absolute favorite is Vol. 7 but for other reasons.
I love all of the workouts and I almost exclusively run outside and modify - I don't always have hills handy when I need them
My recommendation if you want shorter workouts, start with 2 or 3 and see how it works, otherwise, get 5 and 7 and just do one less challenge if you need to keep it short.
Let us know what you think.