It's always a bit of a shock to find yourself unexpectedly pregnant. Give yourself some time to get used to the idea and I have no doubt you will be excited and happy about it!
As for age, it's not super common to conceive naturally at 45, but it sure does happen. My lovely co-cl, PamelaBee delivered a beautiful healthy baby girl at 45.
As for increased risks, yes those of us over 40 do have a slightly increased risk of things like DS, but it's still pretty rare.