A local tri coach suggested http://www.briancalkins.com/HeartRate.htm to me, which is better than the 220- formula but not as good as getting it tested properly. I've used that as a best guide for now - trying to learn to run slower, as I've tended to just go and bash out my training runs before. Getting sick of getting my Garmin beeping at me, but that's what I need.
And if you are wondering who the two lunatics at the half-way drink station between 6pm and midnight were with the lights, that was Mrs Paperman and a friend. They thought they'd try and cheer people on a bit!
I've gone for that as well - I'm aiming for "just finish" to see if I can a least complete the plan without missing too much out - if I'm feeling bored, I'll do what justplodon is doing and filter in a bit of the intermediate one. What are you guys doing for the two plan races?