Eric asked what sort of bikes I ride, and my opinion of them, so here goes:
Giant Iguana Mountain Bike: This is my beater bike, fully fendered, with racks front and rear. I have a custom 7 speed rear cassette I've put together out of a mix of SRAM and Shimano cassettes, starting with a Megarange (34t?) sprocket and going down to, I think, an 11t sprocket. Multiple lights on it for winter use (it's known as the "Starship Enterprise" down at the bike shop). Tyres depend on the season, I currently have
Schwalbe Snow Studs on there, I'll soon (hopefully) be switching over to some 1.75" (ish) cyclocross semi-slick tyres, then when the good weather hits I'll be on 1.5" slicks. It's not a particularly comfortable bike for me, but no 26" wheel based bike is - I'm a shade under 6 foot 5, so all mountain bikes need a long stem and seatpost. I found some extra wide riser bars helped, too.
Giant Cypress: 700c based hybrid bike that I use for long distance touring. It's built up with extra low gears at the front and rear, handlebars and bar-ends are double-wrapped with cork tape, fenders and racks front and rear, and a fully sprung Brooks saddle. This bike will take a massive load; I did a trip up to Truro last year where the final weight on the tyres was 375lbs. It's usually overkill for commuting as it's geared so low, but it's good for going shopping with, especially pulling all the groceries up through Fairview. It's a very comfortable bike for me to ride, and I've done some pretty long distances on it; I think the furthest was just under 200km in a day.
Nishiki Olympic: This is a sport touring style steel road bike that I rescued from the trash. It was in
pretty rough shape when I got it as it had obviously been sat in a garage for several years. I managed to restore over winter, and it came out pretty well. It started off with 27" wheels that got trashed when a driver pulled out of her driveway in front of me, and now it has 700c wheels. It's got cyclocross brakes on the top of the handlebar, and a mountain cassette on the back. No fenders or racks, but it has a handlebar bag on the front to carry stuff in when I'm riding longer distances. It's a fast bike, I can pretty easily keep up with traffic at around 50km/hr going along Barrington.
My general recommendations would be, primarily, to have more than one bike. Designate one of the bikes as a beater bike that can be used in all weathers, and have another bike for nicer weather. If you're riding hills (and at 12km a day, I would guess Eric is) a geared bike is pretty much required - at least if you want to keep your knees through the next decade, or avoid walking up all the hills. I found road cassettes to be too narrow on my road bike (since it has a compact crank set and I ride a lot of hills) so a mountain bike cassette did the trick - note that a long reach derailleur is probably required, though. The age of a bike doesn't have much bearing on the quality, and (for commuting purposes, at least) I'd say the older the better; newer bikes tend to be more lightweight and, as a result, more flimsy, whereas older bikes (especially steel bikes) are a lot harder wearing. The flip side is steel is heavier and rusts more easily.
My recommendation would be to go to your local bike shop and try out a few bikes to see what feels best; I'd definitely recommend trying out different types (mountain, hybrid, road). I know
IdealBikes are happy to rent out most of the bikes they have for sale, and I'm sure if you find one you like and you take it out for the day they'd be happy to take the cost of the rental off the purchase price of the bike.
