An existing condition doesn't raise the premiums, it just isn't covered and you can't claim for it. So any treatment it will ever need isn't insured. Whether that would be just that elbow or anything arthritis or joint related I couldn't tell you though.
If she has arthritis in one elbow at that age, it's likely that either there's something slightly off in that elbow or that something has happened to it...so it's not inevitable that she'll develop arthritis in other joints, but, it is more likely, partly because she'll compensate for the dodgy one and it can impact the rest of her.
Impact...well dogs aren't hugely good at limiting their own exercise, so you'll need to work out what's enough but not too much and be strict with it. Swimming is good exercise without putting strain on it though and labs usually love water.
You'll want her as lean as possible, verging on too thin as that really does make a difference...but she's a lab so be prepared for her to tell you she's dying of starvation, lol.
With elbows, jumping down is really bad, so you want to keep her off furniture so she's not jumping down from it and jumping from a car boot is really bad, also stairs aren't great, so no running up and down stairs all day.
You might want to think about joint supplements as well.
Also - even if her other joints remain fine until old age, that elbow will more than likely get worse, so you will be looking at some point at daily medication rather than occasional, which won't be covered by insurance...it's not hugely expensive as medication goes, but just so you're aware of it.