1. I dunno how your HRM works, sorry. But Friel's intent is to figure out your average HR for the 20 minutes.
2. Look at it this way: it's a time trial but there's no entry fee, nobody watching, no money on the line. If you go out too hard and blow up, you'll know it and can try again another day. If you ease into it and finish with some left in the tank, you'll be wondering if you could've done a bit more.
80 minutes is kinda tough to work with - have you done intervals in the 5 minute range? WTH, try 162 or soo and see what happens. worst case, you'll have a good interval and you can take 5 and repeat.