Worth having a look through my trip reports for the last 9 months - I have been in Wales a lot in that time. Mind you, they are nearly all hill-based rather than level walks.
In the Brecon Beacons, my favourite of the high mountains was Black Mountain (in the west - not to be confused with the Black Mountains in the east). Parking at Llyn y Fan and doing a circuit of the lake will give you some of the best scenery in the area. I did the following, but you can shorten it a lot to just include Bannau Sig Gaer and Fan Brycheiniog:
http://www.walkhighlands.co.uk/Forum/viewtopic.php?f=16&t=20576Nice lower hills in the area are Sugar Loaf and Ysgyryd Fawr:
http://www.walkhighlands.co.uk/Forum/viewtopic.php?f=16&t=19295http://www.walkhighlands.co.uk/Forum/viewtopic.php?f=16&t=21450Pen y Fan is a good hill with very good paths up it but very busy. Also worth a look is Lord Hereford's Knob in the Black Mountains. This is fairly high and has good views, but you can drive to within 20 minutes of the summit:
http://www.walkhighlands.co.uk/hewitts/twmpaI think there is lots of scope for nice lower level walks in South Wales as well, but I can't tell you much about that as I haven't really done any.
In Snowdonia, 10-14 miles would cover a lot of ground. Most of my walks in the area have been much shorter. I'd suggest visiting Tryfan and the Glyders (plenty of reports on here). A circuit of Tryfan. Glyder Fach, Glyder Fawr and Y Garn is a great introduction.
http://www.walkhighlands.co.uk/Forum/viewtopic.php?f=9&t=6781You could do a full traverse of the Carneddau, like Foggieclimber and I did recently, or a shorter circuit of the southern (and most spectacular) summits:
http://www.walkhighlands.co.uk/Forum/viewtopic.php?f=16&t=21137http://www.walkhighlands.co.uk/Forum/viewtopic.php?f=9&t=12878The Snowdon Horseshoe is the obvious classic, and there are also many route variations:
http://www.walkhighlands.co.uk/Forum/viewtopic.php?f=9&t=16096http://www.walkhighlands.co.uk/Forum/viewtopic.php?f=9&t=4311Moel Siabod is much recommended, as is the Nantlle Ridge:
http://www.walkhighlands.co.uk/Forum/viewtopic.php?f=9&t=19310http://www.walkhighlands.co.uk/Forum/viewtopic.php?f=16&t=19906And if you're further south, Cadair Idris is a classic:
http://www.walkhighlands.co.uk/Forum/viewtopic.php?f=9&t=17902There's loads more obviously. Worth flicking through some of the reports of the more obscure hills - there's some gems in there too:
http://www.walkhighlands.co.uk/Forum/hewitts.php?mode=hmap