I decided to go with the option of total independence when I visit SR for the first and only time back in 2012.
Hiring a bike was easy; I hired from a place located right next to the night food market in central SR..... he had recently purchased numerous Giant mountain bikes - I decided against the option of a helmet. Cost was cheap ; around 2 or 3 US a day for 4 days (was bargained down).
As previously mentioned, loads of water should be carried with you and taken at all times - I found the sweet sugar cane drink on ice, also to be very useful for sugar levels. It was very hot but I found myself racing the tourist boats down the river towards the lake and used the bike for the sunset visit to Angkor Wat as well as a full day of temple riding walking (I realise you won't be doing the temples but that is one of the reasons I hired the bike - purely for independence).
I do have history of cycling (though it had been a number of years since doing so). I always found people to be really friendly during my entire Cambodia trip; no matter where you find yourself; or have mechanical problems...there'll always be someone around to help, or begin to walk...someone will help you out.
Have fun and be safe - I was 47 at time of doing this and was smoking 20 a day for over 20 years ; physically you should have no problems :-)
Edit:
If I remember correctly there is at least one cycling club within that area - am quite sure they'd happily take you out to the countryside.