Mildura to Maryborough is 385km by rail.
With an 80km/h line-speed and a half a dozen stops, assuming an average speed from Maryborough to Mildura of 65km/h, you would be looking at about 6 hours. Add the 2 and a bit hours to get between Maryborough and Melbourne and it could be a competitive 8 and a half hours from Mildura to the capital.
All with minimal rail upgrades, and utilising existing rollingstock.
Now lets go a bit nuts...
Assuming the above figures are correct, The current PM departure from Maryborough is 2:57PM, arriving at Southern Cross at 5:22PM. An 8am departure from Mildura could theoretically then arrive into Maryborough at 2pm (allowing a bit of padding in case of delayed running of course) making for a total journey time of less than 9 1/2 hours.
The evening Down Maryborough arrives into Maryborough at 6:57PM (After departing Southern Cross at 4:39PM). A Mildura bound train could be scheduled to depart Maryborough at 7:45PM and then arrive into Mildura at 1:45AM. This probably wouldn't be too appealing, not really what we want if we want this to be a success.
So, maybe instead, the Down Mildura could connect at Maryborough with the AM Down, (departs Southern Cross 9:16AM arrives Maryborough at 11:43AM) departing Maryborough at say, 1:00PM, arriving into Mildura at 7:00PM. That may be a bit more likely to induce success.
Of course, that would require a cross somewhere along the line, I'm sure that isn't beyond possibilities, last I looked there were several crossing loops on the Mildura line, including 1 either side of St Arnaud, either of which could be suitable for the above times. Although I'm not sure V/Line are too pleased about the idea of pass trains utilising trailable loops, but lets leave that aside for now.
Now, of course that would require 2 train sets to be operational, but if the soon-to-be-made-obsolete SG N sets are to be used, that shouldn't be an issue, there are 4 of them that will soon be without a job.
Wow, I went all foamy for a bit there, didn't I?