I moved from NYC to London in September. Most of the folks I know ended up sharing a flat; however, if you're working in Canary Wharf you can find some nice studios down on the Isle of Dogs / Docklands area, but that's a bit residential. The 135 bus runs from the Shoreditch area through to Canary Wharf; that bus route swath might be a good place to look. London Bridge area is fine, but I think the commute would be more of a pain than it'd be worth. Not sure of your age, so perhaps east London won't be your thing, but I find it to be a fun place to live. (I'm still on the younger-side of middle age but not a big clubber or drinker).
As for food - Mexican is indeed in short supply. However londonburrito.com might help you out. Wahaca actually had some not-bad stuff. Spicy and everything. I say this as someone who grew up spoiled with excellent Mexican food choices. Pizza by the slice is either non-existant or (in my opinion) nasty super-thick-crust stuff. But there are a few restaurants that have opened recently that sell pretty nice crispy-crust pizza.
I was spoiled in New York in that I thrice lived right near beautiful parks; when I go for runs now it's mostly on the street. But there are nice parks; they're just a bit of a trek for me to get to.
Yes, the weather can be grating. But whatever. I love the architecture and the history of the city. The museums are great, and the culture is interesting and more varied that I think it gets credit for. And yes, the travel out to Europe is great. In the four months I've been here I've visited Barcelona, Dublin and Paris, and taken a couple of day trips out of the city. Berlin, Bruges, Amsterdam and Edinborough are next on my list.
Basically, I find that in this case, like in most cases, it'll be what you make of it. If you come here assuming it'll be too expensive to have a good time, that the food will suck and the women will be unattractive, that's what you'll find. But if you come here thinking you're going to have a good time, see things you've not seen, and meet up with cool people, then that's what youl'll find.
If you have any specific questions about any of the logistics of being an American in London (like the pain that is opening a bank account here), feel free to pm me.
Best of luck!