One Month to Build Endurance
I'd say you could go with HIIT (high intensity intervals) on a treadmill.
For example a particular training I like and find helpful is:

6-8 sets of:


1 minute very fast (18km/h for me)
1 minute middle speed (12km/h in my case)
1 minute slow (8-9 km/h)


and no stops in between.

That's a very good yet pretty hard training. Do not more than twice a week.