In running there is a rule of thumb that a well trained endurance athlete will have to slow down by ~16 secs/mile to go twice the distance (elties are even less than this). Horwill defined it I think.
So if your PB paces for 10k, 1/2m and marathon get progressively slower by more than this e.g., 10k → 1/2m 40secs/mile, 1/2m → marathon 50secs/mile then your endurance is poor and you should probably train to specifically address it.
What I’m interested in is the equivalent, acceptable “slow down” factor in cycling. I’m presuming the formula, or factor, would apply to Watts instead of speed.
Anyone know if there is one ?