One of next summers riders, Graham Parks crunched some numbers on the rough sections and posted them on the facebook group. Here is what he wrote.
“Cross-referencing comments on YACF with various maps and street view, these are the off-road sections for the 300 and 400 (mileages are for the 400) and my estimates of the length of the unpaved sections:
1. 54 km Cors Caron nature reserve (5.2 km, 24 m ascent)
2. 69 km Claerwen Reservoir path (13.3 km, 154m ascent, includes unbridged ford)
3. 94 km Elan Valley cycle path to Craig Goch dam (2.7 km, 51m ascent)
4. 113 km NCN 818 Cwmystwyth – Llangurig (2.5 km, 131m ascent all in one go)
5. 202 km NCN 82 before Dolgellau (10 km, 380m ascent, starts with 300m climb in 3.5 km)
Total unpaved = 33.7 km”
The TINAT team feel this is about right. Use this information plan your rides.