If it wasn’t for the supply shipments to Britain, they likely would have lost before things got close in the first place. Note it wasn’t just weapons but also food and medical supplies.
Edit: Also, the shipments were regularly attacked by German U-boats, so there absolutely was risk to American lives still.
Aside from the fact that no sane person would mock you for trying to better yourself, you could also start smaller and just, say, go for walks or swimming at a local pool or other not-gym forms of exercise.