A couple of interesting ideas here.
1. Edit costs of inividual technologies - time consuming and you may be forever trying to balance your changes (I tried this).
2. Bottlenech Technologies - nice idea, but would seriously cripple technologically backwards civs (once you're past a bottleneck you'd have a very long time to research better techs and use this to your advantage. Worse, if you're behind and stuck at a bottleneck and someone else gets an advanced unit, how do you stop them overwhelming you before you get past the bottleneck - very difficult).
3. Change Era % - good, simple way to do it imo as long as you can get the balance right. I confess I thought that this value was simply for which era you started in, but may be wrong.
4. The way I've done it is to change CIV4GameSpeedInfo.xml by adding "Transmarathon" - exactly the same but with <iResearchPercent>500</iResearchPercent>.
This will simply extend the research cost, but be careful because you'll find the early technologies very long to research (more of an issue on higher levels where the AI starts with many techs). I counteracted this by getting the palace to provide more commerce, so that initially you'll have same research times but these will slowly and gradually increase. The benefit of doing it this way as opposed to #3 is that you can research your initial techs in time to use them, and it's a smooth transition from ancient to classical era which you wouldn't get with #3.
I can upload my edited files if you're interested. It definitely gives a longer game while keeping the very early game as balanced as possible.