Accumulating runway before quitting your job

My co-founder and I have been developing a Saas product and validating with early users. We plan to quit our engineering jobs in 4 months to do it full time.

For those who have been in a similar situation, did you

a) Save aggressively to achieve your target runway number or
b) Create a business/investment that would generate the cash for your upkeep while working on the main product or for the ballsy ones
c) just wing it and quit without giving it much thought.

I'd love to hear your experience and/or advice on this