"Now or never" v. reservedly

I saw this sprayed onto a road around the Great Orme which is a narrow road around a large peninsular at Llandudno, North Wales this morning. The message was for the rally car drivers who used the road the day before but it made me think about yesterday's cross country race. 

I had the best cross country race of my running career so far. I've judged that on my pace for the course which was an average of 5m43s. I also based it on my position. I approached the race slightly different in mental terms; almost a "now or never" mentality. When running around I kept trying to focus on speed, I was trying to go fast in the moment and not reserve anything for later because I needed to trust and have faith in my endurance. It's tough to convince myself to run fast, I managed to do it for the first mile and then lost focus, then regained focus then lost focus again. I wonder what makes me loose my focus? Is it being not fast enough or fit enough? I don't know. 
What I would like to try one day is a real "now or never" race. To not reserve anything for the miles ahead and just to run at 5m/m for as far as I can without having the finish in mind. I somehow trick the mind into thinking each 100metres of the race is the only 100metres. Is it possible; no it isn't because we all run reservedly because we don't want people to say "he went off too fast". 
Maybe I will go too fast and take a chance at my next race. We'll see. Has anyone else gone off without holding back and had a pleasant surprise? Is it worth doing once in a while?