I think like Dave. Good running shape, which I think I hold throughout the year, is not peak racing shape. I could race a marathon pretty much anytime, but I don't think I could race a good marathon all year around. Staying in shape for that would lead me to burnout.

+1

Maintaining a fairly consistent mileage level year round is fine, but including the type of intensive speed training or focused race specific training is a lot harder.  Isn't that why we often select only a couple races a year as major goal races?  The rest are more like hard training runs to test or evaluate our fitness, or just for fun.