Or...you can learn scales, arps, and other runs as you need them.

As you write the music.

Say you are playing a tune in the predominate key of E or E minor.

You of course want to use scales for leads that are in the key of E.

Always good and expected to play leads over a chord or in a key
that the main backing track is in.

(Now...if you get really INTO music and have listened to more than
just popular music...and want to do something like that music on your
guitar, then there are other possibilities, and things you might try.)

You also want to use scales in the key of E's "partner" keys.

A and D work well with E...in meaning they don't clash like say
say a G# scale would against an E or E minor progression.

You may one day write something that you do want to clash...
who knows?

(Steve Vai is currently getting inspiration from some kind of Bohemian
folk players...strange stuff...but it's NEW and ORIGINAL.)

Throw a major scale against a minor now and then as the resolvences
contrast each other greatly. Diminished scales break up the major
and minor expected seqeunce. Try them as well.

Like for instance in a tune you want to express a moment, make a
phrase that seems to give a feeling of brief hope,
(major=happy/hope...wheeeeeeee...I am so happy)...but then come
back a phrase or two later and put a black cloud of doom on that
Mr Rogers Neigborhood hope. (Minor=Sad...sometimes dark.).

HTH...and as I said...good luck.