Just out of interest, what sort of averages do 100k riders do ? I'd love a go at some longer rides but would have to go really steady to have any chances of making it, and what about stops ? Are there any usually and for how long ?
I have a 100k planned on paper running from Gloucester to Bristol then over the Severn bridge and loop up round via the Forest of Dean back home but it's around double the distance of one of my normal rides and I'm wanting a decent day weather wise to tackle it.
Being Gloucester based myself, I may be able to help here.
Firstly, as your best bets for audaxes to do from here (as they tend to start early at about 9.30am), I would recommend from experience the Three Counties Two Leaf Cloverleaf from Wickhamford near Evesham (August) and the Windrush Winter Warm Up from Ashton Keynes (always seems to fall on Superbowl Sunday). I've heard the Tasty Cheddar is a good one in Somerset too, but you might need a pedalo this year.
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Secondly,The 100km "Three Counties" audax that I did once with a group of six riders took us 5 hrs 30 and we were closer to the back than the front - but not disgraced. The Windrush one, that I have done several times is a 110km ride in the depth of Winter and on average takes me about 6 hrs 25 (about 11 mph inc stops). This year there were 130 riders and at least 20-30 of them still appeared to be out on the road when I finished in 6 hrs 20.(Bear in mind that I'm a bit of a wide-ass chunky monkey and I took full advantage of the two buttie stops, and it might give you an idea).
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Thirdly, I recommend doing the Bridge and Back ride
anti-clockwise, otherwise you'll leave a couple of really nasty hills (Blakeney & Newnham) until quite near the end. Wouldn't fancy that myself. This does mean tackling a nasty climb out of Chepstow, and I'll guarantee the traffic lights will be red at the bottom of the hill! My PB for this one is 4 hrs 49, and I made it 62.9 miles. The prevailing wind usually helps you back home gently this way, as the A38 is nice and wide without much shelter.
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Hope that helps. Good luck.