I was in your shoes last year. Although not as fit. The advice given out in the previous threads is worth reading. I found this site useful too
http://sportivecyclist.com/
Everyone is different, what I did was to do more rides, and get in some longer days, 50, 60, 70. Being localish I went and did a loop riding the three hills, which I think would have been useful if they'd been in. Newlands was markedly easier on the day than it had been on the test ride, no idea if Leith or Box would have been. Knowing where you are on the hills is handy. Everyone climbs differently, I didn't feel happy until I'd experienced them myself, personally I'd rate Leith as the worst of the bunch by quite a way. I did one ride over 100 miles in preparation (around 113 in the end) which was handy because it meant I wasn't upset about the route being cut but also it meant I came in to it knowing I could do the mileage which was a big mental thing for me. I know the training guides don't say to it. I'd definitely do it that way again to take the pressure off on the day.
It's really easy to go out fast. I was averaging 22mph mile after mile through London at the beginning, the roads are great, plenty of other bikes are keeping that pace, it's all pretty flat, you're fresh and full of energy. If the hills had been in I'd have worried about this and probably looked to back it off a little (my normal average is mid 17's) as it is I struggled on Wimbledon Hill (made it, but felt it, must have shown it as I was gee'd on by a guy passing me about 4/5ths of the way up) and it is the sting in the tail, keep it in mind as it's not insignificant and it's 10 miles from the end which adds to things.
I took a café lock, but I never walked more than 5 metres from the bike (grabbing food/water) I don't tend to need to pee when I'm riding so portaloos weren't an issue. Again different people, different takes. There were people around, I wouldn't have left the bike unlocked if I couldn't see it, I may be overly paranoid about that. Removing the garmin is a good move too.
If you can stomach High 5 products (I just used them throughout my preparation) then food won't be an issue. Possibly take some savoury stuff (and train with it) if too much sweet/chemical gets to you. There's plenty of places to recharge though, so don't worry about that too much. Carry a couple of gels on you in case you need them between stops.
Think about how you will get back after. I went for the river boat which was probably a waste, it was difficult to find (hopefully this will change for 2015, they got a lot of feedback about that) and it turned out there was a closed route back to Docklands anyway which would have been fine for me, it's flat and the only reason I went with the boat was that I didn't want to come off of closed roads and start dicing with London traffic. As it was the signposting was so terrible I ended up dicing with London traffic trying to find the pier anyway. If you've done some riding beforehand and got the miles in it shouldn't finish you and riding back after should be fine. You get time to recharge at the end although collecting medals, 'goody' bag and your own kit was very smooth. I had food in my kit bag, stood and ate that, called my wife, basically took 20-30 minutes to let everything sink in before getting back on the bike to set off 'home'. I did just shy of 100 miles across the whole day. I wouldn't worry about being a little way off of Stratford, it's not far and the ride to the start was a good nerves cooler for me. Be prepared for a long day, also the initial getting to the start bit was quite torturous allow yourself 20 minutes more than you think it will take.
As others have said, take what you'll need to fix the normal roadside issues. A couple of tubes and patches would make sense. I had them, I didn't need them in the end, but with it being wet lots of people did, I think I saw a dozen before I got to the start. I didn't go silly with bike prep and ran puncture proof tyres rather than super skinny race things. That worked for me, others will do that differently.
Good luck with it all. It's an amazing thing to experience.
) just once at what would have been half way. The closed roads and restricted access makes it feel more secure than a normal sportive. I didn't take a lock at all and left my strong lock in the official bag (which you collect inside the barriers and after the finish), along with a change of clothes etc. The only time I was off the bike was when it was outside the portaloo 
at the stop. The strong lock was to chain it up for the after-ride pub session! Others' mileage may vary, as it were.
