Never done a race until March this year, I also joined a running club at around the same time and try to compete in as many of the league races that I am able to.

So really to date I have completed one half marathon, one ten miler, six 10k's, one 6 miler and one 5 miler most of which were club runs. I have two clubs runs left, one 12k on Saturday and then one 10k in about two weeks time, so to date that's 10 completed in all wth 2 pending.

On top of that I have signed up to the Birmingham half marathon in mid October and looking forward to the clubs cross country season. I might consider a full marathon next year depends how I feel at christmas time to devote a 3 month training regime along with cross country, etc. 

But to add to the original question, yes it is disruptive to have too many races packed into a short period of time, I completed three in 7 days and kncked my schedule about something rotten. Although that said I do 40+ miles per week, 6 days a week and tend now not to taper for any 10k races and below and just treat it as a hard training session.

I would say for those average trainers training 4+ times per week, a medium distance race of 12k and below  every 3 weeks should be sufficient to allow training and any recovery time, above this say 10 milers upwards then this may need to be spaced out more appropriately, after all we are all not like Eddie Izzard, only Eddie Izzard.

Try to use/treat the races sometimes as a part of your training,especially when the weather is not so good.