Bringing it to a higher level

It's my first time trying out the insanity workout today. Yes, first time. My 21km-marathon-stress had motivated me to increase my cardio level. My colleagues were asking me when am I going to start training for my half marathon. It's insane. I can only do 15 minutes of the first workout and I gave up. When I heard the instructor says 'Here's the suicidal stage'. I thought I will manage to pass through it but I gave up after 40 seconds maybe? I guess I need to get some glucose drink before any workout. Afterall, I got 2 months to train before the marathon. Time is ticking and I gotta do something about it. =)