I'm uncertain about the swimming part so my first foray into a multi sport event is a Duathlon that I will do Mar 5th. This event has a 5k run followed by a 20k bike ride followed by another 5k run. I have found the running part the hardest, but as my weight has dropped (from 298 to 218) the runs have been getting much easier. I now have no problems running 8 to 10 miles.
I still weight train 3 to 4 times a week but do not use heavy weights. I am not trying to build muscle mass just keep what I have toned.