oly tri sept 13th- first one

Posted a few weeks back if you active folks thought i could do it and feedback was 100% go for it (have done 10 marathons, one a year, am comfortable in the water and have been cycling as I was advised to). Also have gotten some great advice on transition hints (although I had no idea what T1 T2 was but figured it out). Have now been doing two workouts a day, bike hour and a half swim 30 min to an hour. Today ran for three hours (training for venice marathon in october). All that being said, isi it true that the actual triathlon is so much more difficult? I heard in this one there are 1000 participants (Mightly Hamptons). Swimming in a pool, riding alone, running alone is so very different.