I've flown United to SF a fair few times, last time was this July. I assume you are leaving on a morning flight from Heathrow. There is very little economy on the plane and if you can afford it I recommend upgrading to economy plus, it's not usually that expensive and the extra legroom makes the flight much more pleasant.
I find United in flight service more relaxed than UK carriers, after take off there will be a drinks service, you have to pay for alcohol, but they have a good range of free soft drinks, then after about and hour to hour and half you will get lunch, recently this has been a chicken dish or a beef dish, a garden salad and a brownie or similar. If you are nearer to the back of he cabin it is unlikely that you will have a choice. There is another drinks service with lunch the they clear away, after this the cabin tends to go quiet with many people resting or watching TV, to try to overcome jet lag I try to rest after lunch as it would be night in SF then be more active for the second part of the flight. You should have decent in flight entertainment. During this period of the flight the cabin crew spend most of there time in the kitchen area, passing through the cabin from time to time with water and orange juice. I always take a bottle of water and a small snack on board so I am self sufficient, but you can go to the kitchen area if you want a drink. About an hour and a half before landing, the cabin comes back to life and lunch is served, this will either be a hot wrap (which I don't like) or a turkey and cheese roll plus a packet of crisps and a chocolate bar.
I don't know about crochet needles sorry, if you decide to pick exit rows they do have extra leg room but the seats are narrower because the seat tray is in the arm.
Just one last thing, sorry this is so long but I hope it helps, cabin baggage on US flights is 1 piece of cabin baggage and a personal item, US citizens take this to mean the largest wheeled case they can get away with and a rucksack or large handbag. What this means is that there is not enough overhead storage space, if you are travelling on a economy ticket, and are not a member of the United loyalty programme you will be in one of the last groups to board and there maybe very little storage space left, even back in economy as the large cases are put wherever there is space, not where their owners are sitting.
Finally I am always cold especially on long haul flights which I find draughty.