Full road bike with drop handlebars seems to be the way to go but I started off on Mountain bikes and when I did change----I had problems. First of all get the size right. Too large a frame will cause problems but so can a frame that is too small. In changing from the upright position of an MTB- I went one size too small and it did cause problems. Back ache and I did the usual trick of raising the bars. THAT DOES NOT WORK FOR EVERYONE. I found out later that the longer lower position is better for me.

But I could not get into the drop position. 10seconds and I was in agony. Had to use the drops for downhills to be able to cover the brakes properly but it was not comfortable. So I practised on the ride getting into the drops. On the flat and I was down. Gradually the pain got less but it did take a while to ride with my head between my knees. This was on one of the "comfort" frames in a Giant OCR but that bike was never really comfortable.

1 year on and I had outgrown the capabilities of the OCR and test rides and shopping got me a full race spec lightweight aluminium frame and forks to build up. The shop built it and set it up-with the bars 4" below the saddle. Had my doubts but that bike is comfortable. Correct size frame and correctly set up for my body that had adjusted to road riding.

On sizing--I am the same as TSL. Top tube length is the critical measurement- Not frame size. I use a 535 and 5mm either way is comfortable but no more. In fact on my 3rd road bike I did not look at any measurements except T.T. length and sorted the bike from there. The mere fact that it was another race geometry frame and the bars have inched themselves down to 4" below the saddle is co-incidental. + the fact that I test rode that frame before I bought it.