I don't set a target mileage I just aim to enjoy my pedalling, be careful if you set a target you might find yourself riding just to make the target and not enjoying it.
I know what you mean Dave, but I am one of those procrastinators who can always find something to distract me when I would really rather be going out on my bike. Today was such a day. I wanted to be out in the sunshine, but ended up doing boring household stuff including putting up new curtain poles. I should save jobs like that for rainy days when I do NOT want to ride!
So, a few years back I started setting mileage targets to encourage myself to go out on the bike more. 2012 and 2013 were scuppered by illness, but I managed to achieve 99% of my target last year (3,619 miles out of 3,650 (10 miles a day average)) and am already at 81% of this year's target (3,243 miles out of 4,015 (11 miles a day average)).
I still only ride when I want to, but when I am out on the bike, I always try to do a few extra miles to boost my tally.
I am 60 next year and have decided to set myself a significantly more ambitious target of 6,000 miles - 100 miles for each year of my life so far.