Sounds like a sensible starting point.
You'll probably get as many opinions as there are people, but...
I'm using a 5 litre pack containing a 1 litre drinks bladder. As well as water, I carry food (malt loaf, fig rolls, jelly beans, Kendal mint cake), tissues, mobile phone, keys, emergency money, space blanket, lightweight emergency warm top, lightweight breathable waterproof coat, lightweight windproof trousers, one blister plaster, one ordinary plaster. I also wear/carry hat, gloves, windproof running jacket.
I'm intending to get a 10 litre or possible up to 15 litre pack, because the present load fills my pack and I'm going to need to be able to carry two litres of fluid on 4-5 hour long training runs on the NDW in the summer. I'll also be adding a compass and whistle (and map).
Before I got the jacket (Montane Minimus) I carried one of those "single use" emergency ponchos - totally waterproof, very light, and useful to avoid getting soaked and cold if e.g. you've turned an ankle and are waiting for someone to come get you.
Note: I carry this stuff on practically all my runs (not for e.g. parkrun) so I barely notice it now. You don't want to wait until the day of your long run then set off with a pack on for the first time. I got used to carrying it from when my long run was under 10 miles.