If some does decide to go to the gym having weighed up all their options, whether they actually do it will be affected by how easy it is to get to the gym, whether the gym is open, whether they can find it and then use the equipment. In other words, it will be affected by their capability and the opportunities they have to put their good intentions into practice. Examples of techniques to remove potential barriers include gyms that are open for 24 hours a day and having gyms in workplaces, to cut down travel time.
Although behavioural economics and nudges are traditional conceptualised as influencing choice, they perhaps can be seen as affecting the myriad of micro-decisions people make when implementing their core choice, i.e. they see the big picture and think that the benefits of an activity outweigh its costs, but actually going to turns out to be too difficult in the moment, something that hyperbolic discounting describes well.
What Works Behaviour Change