Wanted to add:

It does indeed take a fair bit of time to get used to an aggressive position.

I've never ridden fully upright like a comfort bike; I was used to mountain bikes. My shop left my steerer full length (allowing bars to be in my "mountain bike" position just above the saddle to start). I'm dropping spacers slowly as I get used to the bike and limber up (stretching multiple times a week now). By slowly I mean ~3 months and 1000 miles before I dropped the first 5mm spacer out and I was sore after that ride (you'd be surprised how much difference that little adjust made). Good again now, but it will be awhile before the next one comes out.

I can only imagine how much going straight from an upright comfort bike position to a road race position would hurt!