There are no safe easy options. Try building up low intensity running or cardio at the gym for 90 mins at a time two or three times a week (even if its jogging and brisk walking) - gets your metabolism going 24/7.
My plan tells me 8 miles at race pace for Sunday and two 2 miles tempo runs during the week plus a mile wsrm up and down either side. My legs don't seem my own since I drastically reduced mileage two weeks ago. I did and complete to plan a fast tempo run last night and although I wasn't out of breath my legs didn't want to turn over quickly enough. I think I've lost all my fast twitch muscles!