My brother lives in Porthtowan and we rode Porthtowan to Crantock a few weeks back. Pick a duller day and be polite and we had no issues with walkers at all. The section of coastline is stunning and there is some great singletrack and techy sections. After Perranporth the riding gets a bit sand duney for a mile or two then you have another few miles around Crantock before you get to Newquay and need to head back anyway so i'd call it a day at Perran. We havent ridden south down the coast from Porthtowan yet but it's all pretty steep up and down with lots of steps.
Cardinham is always a good call. Nice, fun fast loop - Only about 7.5 miles all in so good for 2 or 3 laps.
Another option you have is riding out of the back of Porthtowan and aim for Poldice quarry. There are loads of DH runs there you'd be fine doing on a trail bike, just check the runs out first as there are some pretty bigs gaps on some runs. There's plenty of nice singletrack around there too.
Oh, and don't go out with Nicolaisam it'll only end in injury