My mage is in a big (~40 ppl) raiding guild and we've cleared every 25 man raid instance, including 2 drakes (working on 3 drakes now). I've run a heroic with guildies twice I think since WOTLK launched. If you keep saying yes then people will tag you as the "go to" guy for doing heroics, so really just learn to say no and you will be fine. The main excuses I use are:

- sorry, already did that heroic today (true many times)
- sorry, online while mostly AFK at work/doing homework/cleaning/whatever

We clear content in 2 nites now (naxx one night, saph/KT/voa/os/malygos the second) and so most people just play alts or don't login the other days, many alts are (close to) 80 so they are all doing heroics together now anyways. Me, I need to power level my DK alt from 61 to 80, but theres always that one heroic to go visit