Yes, I agree with Footsore -- it's a beautiful walk (IMHO some of the prettiest scenery on the southern part of the ODP, if it's a nice day). And, once you climb up from Pandy to the top of the ridge, it's a fairly flat route until you begin your descent into Hay. Try to allow a full day to do it (i.e. start from Pandy in the morning -- There are places to stay in Pandy; one pub/inn is "on" the ODP, assuming they're still offering accommodation.)

If you get a reasonably early start (9am-ish), you'll be able to enjoy the walk along the ridge at a nice pace; one that won't leave you worn out by day's end. Take along a packed lunch (possibly offered by your accom in Pandy) so that you can enjoy it on the ridgeline.

One word of caution -- if it's a bad-weather day (low visibility, etc.), it may be wise to re-think your plan, and walk along the low road between Pandy and Hay. The ridge can be dangerous in poor visibility.

Best wishes!