Correspondance chess is the best way for mediocres like us to visualize combinations and strategy. After playing here for a year, you will find yourself doing OTB combos that make people envy you, you will probably get a higher OTB rating, then on RHP, doing these combos OTB. (many players have stated this from experience)
The terrible thing about correspondance is, it doesn't prepare you psychologicly for competition. Time pressure might bother you, but remember, in standard chess games you have a full two hours on one game, think about it, if you are playing 50 simul games on RHP, and move once in each of them, and you're online for 2 hours, you, mathematicly, should be more then ok in the OTB tournament.
Blitz isn't that great for learning, but I recommend playing 10 or so 10 min blitz games every week. Blitz helps you learn the openings and learn them fast, but other then learning openings, blitz has no uses towards your OTB improvement. In OTB timed chess, no matter what the time frame, if you spend over 5 min on the opening, you're dead/screwed, and this is a hard lesson for the correspondance player. Blitz will help you finish the opening phase quickly without blundering and geting on with your combination forte.