As an example, the cornwall trip I went on about a month ago was "planned" 3 weeks before. I'd been vaguely thinking of doing so for over a year, and in January I suddenly decided to stop procrastinating and actually go there. Booked the time off work, booked the holiday cottage about two weeks before setting off (snagging an unexpected discount in the process), planned my various journeys during the week before and bought travel insurance the night before setting off (all hail the Internet). The holiday was absolutely fabulous, and was full of awesomeness and awesome nostalgia.
Today I got in from work a bit after half five, and did the usual catching up of everything. Read the weekly geocaching email, saw a new geocache that was under a mile from me, and in less than 5 minutes had printed out the co-ordinates and was out the door. About twenty minutes later I was the other side of the M27, having crossed a bridge I've walked past several times but never ventured over before, and tracked down not one but two geocaches (and being the 3rd to find the first cache).
Tomorrow, if all goes well I'll be meeting friends in Brighton, having only decided to do so um... about two hours ago.
Oh, I've really missed my spontaneous adventures. It's been a year since I've walked somewhere just to explore, just to answer the everlasting question of "what's over there?".