If you are just starting back my suggestion would be to do it gradually over several weeks. Depending on how often you go to the gym you could do one day of what you are presently doing, the next of hills, then an easy day, then intervals ie: speed walking/jogging for a minute then easy walking for the next, next interval a bit more difficult walking/jogging then easy walking for a minute and so on....
Since you are in a gym I would suggest you take a session or two with a personal trainer and have him/her show you a weight training routine. Muscles burn more calories at rest and speeds up your metabolism. And muscles look sexy