I'm sure you'll be fine. The threshold for the speed awareness course is pretty high. You'd probably have to be doing over 79 for them to even bother (Although the radar wouldn't care) and as long as you weren't doing more than 30% over the limit you will get the option of a naughty boy (And girl) course. Over 30% is an instant fine and points (Or court appearance depending on how fast you were going).
As far as receiving the letter, usually within 2 weeks... The police are pretty efficient in sending these out, so I would expect it in a short amount of time.