Cycled it about 10 years ago as my niece wanted to do an "expedition" as part of her international baccalaureate course and younger brother wanted to tag along too. Having resolved a few technical issues (she needed a bike!) we started from Plymouth, day 1 to Okehampton for a YHA night and then finished at Barnstaple for train back to Exeter. The junior team then wanted to do their first night ride back down the cycle route to home in Exmouth. My partner and I did it on Bromptons due to concerns about not getting four full size bikes on the train, which I seem to recollect was only a 2 hourly service.
It was a bit lumpy and with only 3 gears I did resort to some walking. It was enjoyable and a challenging but achievable target for relatively novice cyclists.