Interval Training - Best Next Step

Guys, just a quick and hopefully easy question. I started exercising for weight loss three weeks ago, and the induction program set out the whole week for me, including all cardio and resistance training. I have dropped the resistance on a temporary basis as I need to get below 250pound quickly to do a freefail for charity. Once done, the full resistance program will kick off in tandem with the cardio. At the moment I am concentrating purely on treadmill work, originally this was 20 mins intervals of 2 min 4mph walk with 1 min 8.5mph run. I found this too easy, but could not decide whether to increase one or both the speeds, change the intervals or just do it for longer. I elected for the latter and now do the same speeds and intervals for 58 minutes, which happens to be 8km. Was this right choice for maximum fat burn and/or weight loss or not???