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.