Use lookarounds
I think the simplest :substitute command might be to use lookarounds:
:%s/<\@<!<<\@!/"/g
This replaces every < character that is neither preceded nor followed by another < character.
How it works
It uses a :substitute command (See :help :substitute) to replace strings that match the regular expression <\@<!<<\@! with " characters.
The regular expression is reasonably simple when broken down. It consists of three parts:
:%s/<\@<!<<\@!/"/g
^
1
In the middle is a single < character. This is what we want to match.
:%s/<\@<!<<\@!/"/g
^^^^^
2
Before that is: <\@<!. This is a second < character followed by Vim's atom for negative lookbehind: \@<!, which means that our central < will only be matched if we didn't match a < here.
:%s/<\@<!<<\@!/"/g
^^^^
3
At the end we have: <\@!. This is a third <, this time followed by Vim's atom for negative lookahead: \@!:our < will only be matched if it is not followed by a <.
See :help \@! and :help \@<! for more details of Vim's negative lookaround atoms.