Scream- I find it hard to buy presents for people, especially my wife. I've given up trying to do surprises. I know she likes spas, but chances are I would buy her one somewhere she would tell me is rubbish, once it's too late. I would probably buy you the wrong vinegar or an oil that you'd look at and know straight away that you will never use.
I was given a driving experience last year. I knew that they are FAR too controlling, so you don't really experience what the cars can do, which was the case. For just a little bit more, I could have an ipad mini, which is what I asked for this year. I've already got it, since it was on special.
In the event, I chose a spa day for my wife at Champneys with 3 treatments included- BUT I got her to check it over first. Since it was a November special, she's already done it. Would get a lesser deal for more money nearer christmas.
I must say, your BDF into running is most impressive. I think if you'd just got talked into doing a 6 mile run by your mates, you'd probably be alright as a one off, other than soreness. Where you might experience issues is by the accumulation of damage and this has the potential to really do a number on you.
Put simply, you've got the fitness but not the conditioning. If you're really sure of yourself, just keep one eye on any warning signs. If nothing happens, you can give it all the smug 'told you so's', but if you're vigilant you might get away with just stopping for a while.