Running in the sun

As a newbie runner I have really struggled over the summer with the intense trauma that is running in the heat. I did it though, regularly. I felt like a total warrior out there at 5 a.m. and it was already 25c. The hardest runs were the mid day high heat jogs that left me wondering if I was going into heat stroke. The last few days have been dramatically cooler so the running has been more comfortable. BUT, as a result of the extreme heat I hadn't been pushing myself as hard over the summer. I hadn't wanted to get into any dangerous territory of dehydration and heat exhaustion, a very real possibility in the summer sun. So, although I have run 5K I haven't pushed myself that hard in weeks. I fear that I have plateaued at 3-4km as a standard run 3-5x a week. It isn't bad for a person of my weight and inexperience I feel but I am ready to try to kick it back up a notch. This week I have introduced back into the mix some sprints at the end of the 3-4km. These are great for burning a load of extra calories and building up some better cardio. The one drawback to them is that I suffer from more aches and pains after the fact, particularly in my hip flexors. I figure that the more I work those muscles the quicker they will loosen up and the aches will eventually be gone....well that's the plan anyway.