Font Romeu in April is towards the end of its ski-season - it depends when you go in the month. Was there a couple of years ago from the end of March to mid April - not warm, but sunny, with the odd very heavy fall of snow. You'd need to hire a car while you're there to make sense of it; then you can drive down into Spain for slightly warmer conditions and lower altitude (ie, faster running). But better to go to altitude than fannying about on a beach for 10 days IMHO like so many folks do.
I'll second Biarittz, although when I stay down in that part of the world I stay in Hosegor, a village 20 mins from Biarittz. The area is part of the biggest forest in Europe and the trails are fantastic.
Also the lake in Hosegor is wicked to run round. A pretty accurate flat as a pancake 5 mile.
Not sure about a track but with a car there will be one close by.
The tourist season is from June to August and is definetly worth avoiding. I've been down twice in September and accom is cheap, i've also been in March (and am going back this March) and the places to stay are plentiful.
On another note, one benefit of the Algarve (alfamar / albuferia) is loads of training groups go there. But if you want some peace and quiet and some amazing running then I can't recommend Biarritz more highly.
For a quiet altitude spot - Sierra Nevada in Southern Spain was excellent!