。
It should take about 30 minutes on foot.
Literal
Walk-and 30 minutes [within-で] arrive probably.
で here marks the duration within which the action is completed — 'within 30 minutes,' 'in 30 minutes.' Distinct from で of location or instrument; this is the 'temporal container' use. 着く ('arrive') is the intransitive verb; で completes the frame with 'in [time duration], arrive.'