I overheard someone at a fell race earlier this year saying that they didn’t do long races because they didn’t want to lose their speed. Really?
If they were talking pure, flat out, maximal effort speed then maybe but they had just done a 5 mile race.

don’t overlook the long run
Think about it like this: A good marathon runner can do 26.2 miles in two and a half hours – that’s a pace of just over three and a half minutes per kilometre.
That’s like doing a 36 minute 10k then continuing at the same pace for another 20 miles!
Unless your goal is to work on pure, fast twitch dominant speed then you’ll benefit from a mixed training schedule that includes easy, long runs which help to develop and maintain your aerobic base. This aerobic base fitness is exactly that – a solid foundation on which to build the rest of your training. Of course you also need to train at faster paces if you want to do well in short races but don’t overlook the long run – it won’t slow you down!