sunshine,
Usually A-fib IS a sudden attack that can persist for minutes, hours, or days. It is annoying, scary, life-disrupting, but usually not a killer.
IF on the other hand you are suffering any rhythm irregularities caused by starvation then you have damaged your heart severly and dangerously. When anorexia kills, it ususlly does so by fatally interrupting the normal heart rhythm.
Lay your problem out to a competent cardiologist and get a good workup. That "80 year old heart" line was probably just to scare you...I hope. That would describe a heart in actual heart failure.
It's impossible to judge precisely how much danger you are in because some people do a lot better than others....it's a bell shaped risk curve. But you're not likely to drop dead before Christmas.