I will give you a couple methods:
BFL cardio example (treadmill)
Minutes: Speed - all speeds done for one minute after warmup.
1-2:  3 mph
3:  4 mph
4:  5
5:  6
6:  7
7:  4
8:  5
9:  6
10:  7
11:  4
12:  5
13:  6
14:  7
15:  4
16:  5
17:  6
18:  7
19:  8 mph
20:  4
20 minutes.  Each successive time, increase the bottom and intervals (4.2 mph, 5.2 mph etc...)  If you can the base level to 6 mph....you'll be able to run 5.
Or try this:  Jog mile.......run 1/4, walk 1/8, run 1/4 - until you get another mile....then jog last one......
Either one will help you ramp up, without actually running more miles.