Many people are more altruistic than me and don't seem to realize they are doing themselves a favour

If it benefits you, it isn't altruism - at least by the conventional biological definition of altruism.

Here is Trivers (1971) in "The Evolution of Reciprocal Altruism" on the topic:

Altruistic behavior can be defined as behavior that benefits another organism, not closely related, while being apparently detrimental to the organism performing the behaviour, benefit and detriment being defined in terms of contribution to inclusive fitness.