I'm going to regret saying this but the overall gradient of this ride isn't too bad.
Yes, it gets 'gnarly' in some areas but 6202 metres in 600km overall isn't too bad.
(Allow for plotting/rounding errors in Fugawi Maps).
The distance is another thing, I'm not used to riding distances like this but I'm used to average gradients worse that this. Having said that it's worse than the Denmead 300km I just completed.
To put it in perspective the equivalent 600km climbing for the following events would be
Watership Down 100 (1423m) * 6 = 8538 metres
Denmead 300 (2821m) *2 = 5642 metres
Faccombe Haul 100 (1960m) * 6 = 11760 metres
Denmead Mince pie 100 (1259m) * 6 = 7554 metres
All based on Fugawi stats so they should be consistently accurate or innacurate.
The highest point on the ride seems to be about 324 metres, 4km south of Talgarth. This is only 50 metres higher than the roads near me on the Newbury Downs (Combe Gibbet) so it's not like I'll need oxygen either.
It's just those damn miles and the sleep thing.