Pick a mini tri to start with - that was the only way I attempted my first. And I still had to walk part of the 2km run (sorry, can't convert, but that's really short!!)

I used to run a fair bit and have done a couple of half marathons (2hrs) and heaps of 10km runs, but stopped running about 3 years ago after my last half. I was carrying about 6kgs more weight and wasn't enjoying running. My training partner (who goes along with any training plan I come up with ) and I have just started running again. We started off running 1.5mins/walking 1, then upped it to 2mins run/1min walk; now it's 3/1 and we have been doing that for about 50 mins. It helps your recovery and means yuo can go for longer and are less prone to injury. You could try that in training and see how your knee copes.

I'm almost 50 and she is almost 40. And don't be in awe, anyone can do a tri!