A few personal preferences:

Avoid casting regen at the very start of a pull if it's particularly big, if the tank doesn't manage cooldowns well, you may find that by the time you've got a cure going, they are potentially going to be super low as it is, personally I always start with a holy and go from there. The cast and stun buys you plenty of time to assess and adapt accordingly.

Secondly, if you are are allowing yourself to get 20+ yards from the tank then frankly you need to wake up and start anticipating the situation better. You need to be your tank's shadow between pulls, stick yourself 1 foot behind them and stay glued there until the mobs are rounded up. You have sprint, the tank likely doesn't, no excuses! =(

Lasty, WHM is a powerhouse in Brayflox once you get it down. Only BLM can can compete with us for DPS on some of those pulls. No other job has a chance if the tank is geared enough to hang in there and let you use cooldowns for damage. As mentioned, the next challenge is figuring out how to balance your MP usage so you don't become a burden between pulls.

Good luck, speed running is intense and enjoyable imo!