How's the training going Andrew?
I have to agree with Alarm, when you can't run anymore due to age/injuries you really miss it.
Take the downhills very carefully, as wreckedit said, the downhills can be the most damaging on your knees. If necessary, walk downhill and don't worry about the rest of the pack, you'll easy catch them up on the uphills.
You'd normally lengthen your stride downhill and shorten it for the uphill, but if it's too painful/damaging downhill don't risk it. (Same applies when you're walking)
If there's a choice of grass or tarmac, take the grass, it's easier on your knees.
When I used to go out walking the hills and mountains with friends I found I could outpace them uphill but never worried about it 'cos they'd catch me up on the dowhills.
I remember setting out to ascend ScaFell Pike one day, felt good, stepped down off the little bridge that goes over the little stream at the very beginning and called the day off for me. I could have easily ascended but I knew then that it would be painful coming down again.
After Mont Blanc I limped badly down from Tete Rousse to catch the tram whilst friends ran on to delay it. I only just caught the last one of the day, it would have been an overnight bivii 'cos there was no way I could have walked the rest of the way down that day.
Annoyingly, I could have outpaced them easily on the scramble down from Gouter Hut, but it's difficult to pass on that stretch.
Latterly I always took walking poles, didn't use 'em on the way up, just on the way down.