Overweight runner, need help to get out of my comfort zone

I am 6' 4" with 220lbs weight. I have been running since 2006 during summers only. Skipped last year and put on some unneeded weight. I run 4 days a week (mon-thu) in following format...

5k (about 28 mins) -> 5 min walk -> 1.2k

I have been stuck to this format for as long as I can remember. I need to get out of this. I ran 9k couple of weeks back but it rained shin splints and I couldn't do much running for rest of the week. Sometimes I feel that it is not physical constraint but a psychological one. How do you guys grow out of your comfort zones and run longer with increased pace?

Thanks.

Edit: Where I am from it's 35C (95F) this time of the year with ~70% humidity. Tips to stay hydrated would also be welcome.