Relating to my earlier answer... I'm not sure if there should be a hyphen anywhere.... long pined-for picnic ? No, I don't think it is needed. What do others think? a long-pined-for picnic? Looks awful. But I'm having a mental block. Need coffee. I'll stick with a long pined for picnic.

It was a glorious autumn morning ideal for a long pined FOR picnic. One pines FOR something. Examples: the dog pined for his owner; the lady on a diet pined for chocolate cake. One can also pine AWAY and die. Oh dear! One can simply pine. But in your sentence we need the FOR. If you substitute 'yearn' you still need the preposition ... long yearned for picnic.

Apart from the lack of comma after 'morning', the sentence as a whole is grammatically correct. Unfortunately, the phrase 'long-pined' doesn't mean anything, I'm afraid. You can pine FOR something which means that you miss something or someone greatly, but you can't use this word to talk about a picnic that you have been looking forward to. 'Yearn' is a slightly better word in this context, but again you 'yearn for' something, so you could possibly say 'a yearned-for picnic' or 'a longed-for picnic' . Neither of these sound entirely natural, though. If you want an expression with 'long-', you could say 'long-awaited'.

I think that sounds fine. It's more poetic than everyday speech. In fact, a nice picnic does sound pretty nice about now, haha.