Reference: Day's Journey
Easton
The usual length of a day's journey in the East, on camel or horseback, in six or eight hours, is about 25 or 30 miles. The "three days' journey" mentioned in Ex 3:18 is simply a journey which would occupy three days in going and returning.
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Hastings
A 'day's journey' (Nu 11:31 f, 1Ki 19:4; Jon 3:4; Lu 2:44; cf. three days' journey, Ge 30:36; Ex 3:18 etc.; seven days, Ge 31:23) was not, like the 'sabbath day's journey' (see Weights and Measures), a definite measure of length, but, like our 'stone's throw,' 'bow-shot,' etc., a popular and somewhat indefinite indication of distance. This would naturally vary with the urgency and impedimenta of the traveller or the caravan. Laban in hot pursuit of Jacob, and the Hebrew host in the wilderness, may be taken to represent the extremes in this matter of a 'day's journey' (reff. above), although it is scarcely possible to take literally the 'seven days' journey' of the former (Ge 31:23)