Well, its definitely not something you can rush through unless you are very very good or your piece is just fairly simple (I composed this march in one evening).
I'm pretty bad lol, so this march is fairly simplistic.
I only used a traditional orchestra, rather than a romantic/modern one, which is absolutely massive, and would have taken me at least twice as long.
You do need to kind of plan ahead. Think of a really imaginative and unique theme for your composition, and you can recycle it a few times like ABA form or a recapitulation, or with a few variations in it.
Probably the most difficult part for me at least is the middle section, making sure it doesnt drag on and bore the audience is a bit of a problem for me haha.
Also, getting dynamics and crescendos right is something not to be missed out, a piece is very boring without dynamics, so make sure there are plenty of contrasts in dynamics, the crescendos and diminuendos on sibelius are crap and hardly make any difference, so you'll have to manually assign each note in cresc. phrase to a individual dynamic, eg. pp, p, mp, mf, ff etc. This takes longer than it looks, because of all the scrolling through menus.
Also, do take some inspiration from other composers, be it beethoven, mozart, bizet, shostakovich, stravinsky or whoever you like the most (just be careful not to outright plagiarize their work!). They all took inspiration from each other and some were under the tutelage of each other.