This happened several years ago
I had spent two nights at a friends house and as usual I biked there. The bike path is heavily used by locals and tourists alike and runs about five miles from my house to his.
At the sleep over we did typical guy things, talk about girls, play video games and drink amounts of soda that would make anyone else gag. Well at the end of the night I was feeling particularly horny, but I would never masturbate at a friends house unless specifically asked. So I just went to sleep. When I woke up I had the usual 'morning wood' so I decided to say goodbye and leave early.
It was a beautiful sunny day out and as I get on the bike path at around nine in the morning I noticed there was very little traffic compared to normal. As I rode along I realized my dick was partially hard and the breeze going up my shorts felt amazing. Masturbating outside had been one of my fantasies for a long time but I never got up the courage to do it for the fear of getting caught. I usually wait until I get home to masturbate but seeing as I had just spent two nights at a friends house and was very horny and riding a bike with a boner is extremely uncomfortable I pulled off to the side of the bike path.
This area has trees on the right of the path and to the left there's a large field. I took my bike with me off the path to the right so any nosey passerby wouldn't come looking if I left it just off the path. I push my bike over a slight incline then down the backside, I left it there as it was just out of sight of the bike path laying down. I walked further back up over two smaller hills just to be safe. When the bike path was out of sight and I knew that no one riding by could see me I undid my belt buckle dropped my shorts then my boxers and sat on a nearby smooth log. Dick in hand I started slowly stroking as I usually do, the soft breeze on my nuts felt exhilarating.
I picked up the pace going faster and faster my nuts were slapping against the smooth dead tree, my dick was throbbing trying to hold the cum back for just a few more seconds. Then I came more than I had ever came before, eight ropes of come shot out in succession onto the dirt and pine needles below. I sat there breathing heavily for a few more minutes in amazement at what I had just done. I shook off as much come as I could then I put on my boxers and my shorts. I went and found my bike, got on the bike path and rode home.
When I got home I immediately took a shower to clean the stickiness off and masturbated again to the thought of what I had done earlier. I've masturbated outside since then on the same path but a different spot with the same friend that I had spent the night with, but that's a different story.