My daughter had to swim out to a white buoy and then back as far as the jetty at the Coral Hilton (no wetsuit) then do the float/tread. This last bit was made particularly easy because the instructor was there talking to her so the time passed quickly.
I did the swim/float part straight after back-to-back training dives lasting 45 minutes each, so was getting a bit tired (in Greece). Like Everest I'll confess to not being entirely fit - but even so I headed off at what I thought was a gentle paced front crawl and the instructor called after me "it's not a race".
I found taking the scuba gear off at the surface then putting it back on far harder.
One thing I would recommend is that you buy, steal or borrow a copy of the PADI Open Water manual and read it through before you go. Here's a place that sells them (
http://www.divingshop.com/), or try a local dive shop.
Oh dear TAH-M, I got away with doing this swim in a short wet suit. Fortunately I can do plenty more than 8 lengths and tread for at least 15 minutes down the local pool so I won't have to send the PADI card back...