Marathon mode should use an automatically-adjusting Elo system: initially, all marathon mode games start with, say, 10,000 HP, but it shifts down as players win games with that amount of HP, and shifts up (probably more slowly) as players lose games with that amount of HP.

Rather than marathon mode using a fixed value of 999 HP, you can set an option to control how much HP is used. (If the option is not set, it will default to 999). The value should appear in both the xlogfile (so that tournaments can set some threshold of HP beyond which the game doesn’t count) and dumplogs.