Hey, don't take this the wrong way please, trying to help.

1) Con loss is not nothing, but it's nothing major either. The level gains take into account your current, adjusted con. Thus, if you have a max of 20 con and your current con is 18, wear an item or two for +2 con (very easy to find, also known as anti-rot/malice set to pretty much anybody) for when you're about to level and you're set with max hp gains. (Same goes for other stats BTW).

2) There are trains you get every 5 levels plus 10 pracs can become a train. Some use it to train hp, some save them for con loss recovery, it's a personal preference.

3) Leveling to hero CAN be done in 20sh hours, but that would involve some fairly special circumstances and being lucky with the groupmates. My chars (and am a vet of 20sh years) usually take at least 50 hours to level to 51, but more common (for me) is 100sh. Granted, some of it is the desire to get adjusted to the build, sit at the lower ranks or sweet spots at least for a little while, practice key skills, etc, but it would be rather challenging for me to get to hero in 20 hours with the majority of the builds. And even then, it's probably not a good idea as I'd have 77% in most skills/spells, which gets you killed at hero rather easily.