Preview play ahead of the green thumbnail cache line should be smooth. When that completes, green is above all video clips but not image clips.
As for the ribbon under the preview window...
Blue color indicates the sequence is not cached. Purple color indicates the sequence is cached as a result of mixing multiple tracks. You will not see purple cache if the sequence only has one track. Gray color indicates the caching for full screen preview which is the sequence output in full screen size.
Often, it's not necessary to wait for caching to complete for successful export. If audio isn't fully cached there should be a warning and the waveform won't appear. And there are instances when the green line has completed, but audio is still processing. A counter may appear on the clip in the bin.
But your mammoth project may act differently. Try breaking it up - perhaps into three-hour chunks - exporting each, then assembling those clips for the final.
And use the 64-bit Videopad version. For licensed users upgrades are free for up to six months from purchase date. After that, VP will continue to fully function but a fee will be required in order to register the newest. Retain your old install file and registration info.