EBNF of the grammar

parts = part, { "+", part };
part = repetition | step;
repetition = number, "*", "(", parts, ")"; 
step = time_step | distance_step;
distance_step = distance, effort;
time_step = time, ("min" | "s"), effort;

EBNF can be rendered with PlantUML

Implementation always normalizes to a 1 * (<input>) which is a bit of a hack since we basically start parsing with a repetition instead of parts.