I always found that when if I wrote longhand first, when I came to typing it up either on a typewriter or, later, a computer, I'd have little motivation to "rewrite stuff I've already done".
I'd edit on the fly, paraphrase, revise, add stuff, ignore other stuff and then eventually stop entering it into the computer.
So now I do everything I can on the computer except for those occasions where I have no access to one and come up with an idea. I still have the problem when it comes to putting my idea into the computer but it's smaller than a chapter so I generally get it entered before hitting the "give up" stage.
Now, I have the additional problem of Carpal Tunnel Syndrome, which means gripping a pen(cil) for any length of time is painful and then makes my fingers go numb - I can't even hold a telephone or use a normal mouse for long periods, let alone grip a narrow writing implement (the narrower it is and the more pressure it needs, the faster the CTS kicks in) for any length of time.
I don't get the same issue with typing, especially now that I've learned to touch type (for which I use Dvorak) rather than multi-fingered hunt-and-peck. I also use a trackball so when I do have to use the mouse, I can just rest my hand on it and roll the ball with my thumb rather than actually grip anything.
Consequently, it takes much longer for my hands/wrists to ache typing than doing longhand.
Using a word processor is not without its own problems - primarily the ease with which I can get distracted into revising, refining, editing what I've already typed instead of working on writing the rest of the story, but at least I no longer waste time writing pages and pages of stuff long-hand only to give up on entering it into the computer part way through.
If I've written it, it's already in there.
I do relate to the OP's issue of thinking of stuff when I'm miles from the computer and then being unable to write it down once at the machine. Done that too many times to count.
The words often do flow better and faster while I'm out and about doing stuff, or even just sitting outside having a cuppa and a ciggy, than when I'm at the computer and I find it hard to recapture what I thought and put it in at a later time - but often the times when I get my best creativity, I'm unable to write it down long-hand either as I'm driving or riding or doing something else.