Training strength and endurance seriously at the same time, rather than treating one as an afterthought to the other.
Also called concurrent training in the research. The concern is the interference effect: endurance work can blunt strength and size gains when the two are stacked badly. The effect is real but smaller than it was once thought, and it is mostly manageable through scheduling.
Four rules cover most of it. Separate the sessions by at least six hours, or put them on different days. Put the priority quality first when they share a day. Favour cycling over running for the cardio, since the eccentric load of running interferes more. And cap the hard endurance work at about three sessions a week.
A workable week for a client who wants both: lift Monday, Wednesday, Friday; zone 2 cycling Tuesday and Saturday; one hard interval session Thursday.