If you dont want to knacker yourself Id take this advice. You can still go swimming, cycling etc but you need to ease down on running or face the consequences. Could also cut the distance down and just jog on grass to warm up down after a cycle
Another idea is to get a small round trampoline like the one used in aerobics and run on that which will have less impact. Swimming lenghts and cycling will keep your stamina up. As previously said its harder to come back after an injury so prevention is best.
Nothing will help your 1.5 mile time like running, but anything that gives you a good cardio workout (i.e. gets your heart & lungs working hard) will be some help. Also, you will run faster if you lose your gut (no offence - you mentioned it), & swimming & cycling & circuits, and many other types of exercise, can help in this.
What time can you do now? What time are you hoping for?
also hit the weights, with the right lifting routine you can improve your running performance*, somthing about stregthening the muscles in your shoulders and back to help your breathing. Improving the circulation in your upper body and doing squats/leg presses etc to built up musle in your legs that is usually forgotten by running cause stuff like runners knee or somthing like that.
this is just info i have recieved from various sources, it would proberly be best to go see a medical pro ... but a most people never bother ...