Would it help if you had some music that you really loved to whack on really really loudly to get you pumped up to exercise?? I tend to be a bit unmotivated with housework, especially if I'm not in the mood and I find it helps if I put on some music I can sing and dance along with...don't know if that will help or not?!?
Make sure you have a good pair of joggers too, if you haven't already, go to somewhere like the athletes foot or a proper sports shop and get fitted correctly for a pair of running shoes, having the right shoes to provide support where you need it will make a world of difference. i used to do lots of cross country running when I was younger, back at school, I did have orthotics but could never wear them when running as they would give me bad blisters, so I a correctly fitting pair of shoes was best. As for the heat..when do you exercise? If you can get up and do it early in the morning before it gets too hot, even when everyone is still in bed! Then jump in a nice cool shower and feel nice and refreshed and ready to start the day. Maybe vary your routine too, instead of jumping on the treadmill day in and day out, can you go for a long walk around your suburb or in a nice park somewhere? Change of scenery also does wonders!