I'm also a runner OP and running is FULL of older men trying to meet younger women. I always try to avoid them, because I find them annoying. I'm not suggesting for one moment that your boyfriend is annoying, just describing my experience. They always try to make some excuse to come up and talk to you, and to be honest, I invariably find them patronising or boring.
It also puts me off that they will either have several failed relationships behind them, and know exactly what to say to make you think they are the good guy, OR they are the non-relationship type. From what I've seen over the years, quite a number are the serial monogamist type, going from one relationship with another runner to another, with no intention to ever commit.
I know so many wierdos that are runners. Quite a few that several years ago I thought were the sale of the earth, quite talented type, who ended up losing their jobs because of bad behaviour, flirting with clients, stalking a younger girlfriend when she finished with them, etc. So many cheat on their girlfriends, or have tried to with me through meeting me through running. Its like they try to line up a sporty replacement first before ditching the existing girlfriend. Offering you a shared lift to a race or training session is a particularly favoured tactic.
Then you get the slower type who follow women throughout races, ignoring all male competitors, sticking to your heels, then use that as an excuse to chat to you at the end of the race and latch on, so you can't get rid of them.
Just my experience OP. A LOT of men treat running as a way to meet women, whether or not they are single. I'm very very wary of them, for good reason. There are plenty of women their own age for them to meet through running so meeting a younger woman is the holy grail.
My closest friend, said to me a few days ago that maybe our wedding being called off is a sign, that perhaps I shouldn't marry him. That she thinks I'm young and beautiful and have so much life to live and that I could find someone younger to share that with. I'm just really hurt, why is he not worth sharing my life with because he's older?
Those are quite strong words by your friend. I wonder if she is picking up on things that you aren't. So many of these runner types know that they are in better shape than the average man of their age, and they use that confidence to flirt with younger women.